Gas turbine engines employ fans and compressors with longitudinally alternating rows of rotating blades and nonrotating vanes to compress an incoming air stream and direct that air stream essentially longitudinally. The blades and vanes are airfoils which are disposed radially across an annular flow path between concentric inner and outer cases. The vanes are joined to both the inner and outer cases and have a natural vibratory frequency which is higher than the excitory frequencies to which the vanes are exposed during normal engine operation.
During engine operation, it is normal for different parts of the engine to be at different temperatures. For example, the inner case may be at a temperature different from that of the vanes and the outer case. Moreover, the inner case temperature can vary independently of the temperature of the vanes and the outer case. Consequently, the inner case expands or contracts in the radial direction relative to the vanes and outer case. To accommodate this differential thermal response, the inner end of each vanes is anchored to the inner case so that longitudinal and circumferential displacement of the vane is resisted by the case, but radial expansion and contraction of the case relative to the vanes is unimpeded. The outer end of each vane and the outer case share a common thermal environment; accordingly, each vane is securely joined to the outer case so that no relative displacement occurs between the vane and the case in any direction-longitudinally, circumferentially or radially.
In one known arrangement for anchoring each vane to the inner case, a longitudinally and circmnferentially extending shroud is attached to the inner end of each vane. The shroud contains an aperture in the general shape of the airfoil cross section of the vane. A sidewall at the perimeter of the aperture defines a compartment extending radially away from the flow path. The inner end of the vane fits into the compartment, spaced apart from the compartment sidewall. A vibration damping material, also referred to as a potting material, fills the void between the sidewall and the vane, thereby gripping the vane and securing the shroud thereto. One or more support pins extending radially outwardly from the inner case also penetrate into the damping material to complete the connection between the vane and the case. The pinned connection resists longitudinal and circumferential displacement of the vane relative to the case, but permits radial displacement therebetween to accommodate thermal effects. When the vanes and shrouds are assembled into the engine, each shroud abuts the circumferentially adjacent shrouds to form a continuous inner flow path boundary. The damping material, in addition to joining each vane to its shroud, also damps vane vibrations and resists circumferential and longitudinal displacement of the vane arising from the aerodynamic loads thereon.
While the above described arrangement is effective in older engines, it is inadequate for modern engines which operate at higher temperatures. Some of the thermal energy which causes expansion of the inner case is conducted through the support pins and into the damping material thereby elevating its temperature in the vicinity of the pins. At temperatures higher than a critical temperature, referred to as the transition temperature, the damping material near the pins becomes too soft to resist displacement of the vane relative to the case and pins.
This shortcoming might be overcome by substituting a damping material of higher transition temperature. However, materials with higher transition temperatures are also more resilient than those with lower transition temperatures. A more resilient material, distributed throughout the compartment, lowers the natural vibratory frequency of the vane. This is undesirable since the vane is exposed to vibratory excitations of various frequencies during normal engine operation. If the vane's natural frequency is lowered so much that it coincides with the frequency of one of the excitations, the vane will vibrate violently during engine operation, leading to the vane's damage or destruction.
The reduction of vane natural frequency could be mitigated with modifications to the vane geometry to make the vane stiffer or by constructing the vane of a different material, thereby compensating for the reduction of vane natural frequency associated with a damping material of high transition temperature. These approaches, however, are likely to increase the weight of the vane, a clear disadvantage in an aircraft turbine engine where weight minimization is a key design criterion.
The substitution of a higher transition temperature material may be undesirable even if the reduction of vane natural frequency is tolerable. During manufacture, the vane, shroud and damping material are subjected to a high temperature curing cycle to harden the damping material and bond it to the vane and shroud. Materials of high transition temperature are processed at higher curing temperatures than their low transition temperature counterparts. These elevated curing temperatures can distort the vane, which in modern engines is usually made of a nonmetallic material. Moreover, higher transition temperature materials tend to form weak bonds with the vane and the compartment wall, thereby compromising the structural integrity of the connection.
In view of these shortcomings, a vibration damping shroud that retains its damping capability in an elevated temperature operating environment without depressing the vane natural frequency, increasing engine weight, compromising structural integrity or risking vane distortion during manufacture is sought.